The present disclosure is related to semiconductor light emitting devices, and more particularly to structures with an alternative to traditional cladding layers, and method of producing same.
Semiconductor laser diodes (LDs) emitting in the range of 500 nm, also known as green-wavelength LDs are of current technical interest for a variety of applications, such as full-color visible displays (complementing the existing red and blue LDs), undersea communication, etc. Although nitride ultraviolet (λ<380 nm), near-UV (λ≈405 nm), and violet-blue (405 nm≦λ≦470 nm) laser diodes have been demonstrated and produced commercially, their performance deteriorates for longer wavelengths. The sources of this reduced performance are numerous. First, longer wavelengths imply an active indium-gallium-nitride (InGaN) region of higher indium content. These alloys experience greater strain with respect to the GaN template they are typically formed upon. The higher strain may be responsible for structural defects that destroy the internal quantum efficiency; and the greater strain is also responsible for a greater piezoelectric field across the quantum wells, which also reduces the radiative efficiency by separating the injected electrons and holes. Accordingly, significant research is being undertaken relating to nonpolar or semipolar orientations of GaN.
FIG. 1 shows a generic nitride laser diode structure 10. Portion 12 of FIG. 1 shows a bandgap-energy representation, and portion 14 shows the corresponding refractive index profile associated with this structure. An optimized LD structure achieves both strong carrier confinement and optical confinement. The carrier confinement is realized by including high-bandgap alloys in the heterostructure, specifically in the cladding layers surrounding the quantum well active layer. A cladding layer having a low refractive index produces strong optical confinement.
The range of alloys to form such heterostructures is limited, however, to compositions that are not excessively strained with respect to the underlying layer (such as GaN). Thus, another challenge associated with forming a green laser diode is the difficulty of achieving adequate optical confinement. This is a consequence of both the smaller refractive-index differences (i.e., lower dispersion) of InGaN alloys at longer wavelengths, the longer wavelength itself (since the mode size scales with wavelength), and the strain limitations that may preclude using AlGaN cladding layers (which are tensile-strained and prone to cracking). Accordingly, described herein is an alternative nitride laser structure where the upper cladding layer is other than AlGaN. Investigations into alternative upper cladding layers has led to the realization that such alternative cladding layers may have applicability not only in the green wavelength devices, but in many other devices such as those emitting in the violet-blue, red, and infra-red region. This disclosure explores such structures and their applications.